1. Field of the Invention
The present invention relates generally to gas turbine engine turbine airfoil cooling and, more specifically, to turbine airfoil trailing edge cooling holes leading into trailing edge cooling slots.
2. Description of Related Art
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. The hot gases are channeled through various stages of a turbine which extract energy therefrom for powering the compressor and producing work, such as powering an upstream fan in a typical aircraft turbofan engine application.
The turbine stages include stationary turbine nozzles having a row of hollow vanes which channel the combustion gases into a corresponding row of rotor blades extending radially outwardly from a supporting rotor disk. The vanes and blades have corresponding hollow airfoils with corresponding cooling circuits therein.
The cooling air is typically compressor discharge air which is diverted from the combustion process and, therefore, decreases overall efficiency of the engine. The amount of cooling air must be minimized for maximizing the efficiency of the engine, but sufficient cooling air must nevertheless be used for adequately cooling the turbine airfoils for maximizing their useful life during operation. Each airfoil includes a generally concave pressure sidewall and, an opposite, generally convex suction sidewall extending longitudinally or radially outwardly along a span from an airfoil base to an airfoil tip and axially in chordwise direction between leading and trailing edges. For a turbine blade, the airfoil span extends from a root at the radially inner platform to a radially outer tip spaced from a surrounding turbine shroud. For a turbine vane, the airfoil extends from a root integral with a radially inner band to a radially outer tip integral with an outer band.
Each turbine airfoil also initially increases in thickness aft of the leading edge and then decreases in thickness to a relatively thin or sharp trailing edge where the pressure and suction sidewalls join together. The wider portion of the airfoil has sufficient internal space for accommodating various forms of internal cooling circuits and turbulators for enhancing heat transfer cooling inside the airfoil, whereas, the relatively thin trailing edge has correspondingly limited internal cooling space.
Each airfoil typically includes various rows of film cooling holes extending through the sidewalls thereof which discharge the spent cooling air from the internal circuits. The film cooling holes are typically inclined in the aft direction toward the trailing edge and create a thin film of cooling air over the external surface of the airfoil that provides a thermally insulating air blanket for additional protection against the hot combustion gases which flow over the airfoil surfaces during operation.
The thin trailing edge is typically protected by a row of trailing edge cooling holes which breach the pressure sidewall at a breakout immediately upstream of the trailing edge for discharging film cooling air into trailing edge cooling slots. Each trailing edge cooling hole has an outlet aperture in the pressure side which begins at a breakout and may or may not be bounded in the radial direction by exposed lands at aft ends of axially extending partitions which define the cooling slots.
The axial partitions may be integrally formed with the pressure and suction sides of the airfoil and themselves must be cooled by the air discharged through the cooling slots defined thereby. The partitions typically converge in the aft direction toward the trailing edge so that the cooling slots diverge toward the trailing edge with a shallow divergence angle that promotes diffusion of the discharged cooling air with little if any flow separation along the sides of the partitions.
Aerodynamic and cooling performance of the trailing edge cooling slots is directly related to the specific configuration of the cooling slots and the intervening partitions. The flow area of the cooling slots regulates the flow of cooling air discharged through the cooling slots, and the geometry of the cooling slots affects cooling performance thereof.
The divergence or diffusion angle of the cooling slots can effect undesirable flow separation of the discharged cooling air which would degrade performance and cooling effectiveness of the discharged air. This also increases losses that negatively impact turbine efficiency. Portions of the thin trailing edge directly under the individual cooling slots are effectively cooled by the discharged cooling air, with the discharged air also being distributed over the intervening exposed lands at the aft end of the partitions. The lands are solid portions of the pressure sidewall integrally formed with the suction sidewall and must rely for cooling on the air discharged from the adjacent trailing edge cooling slots.
Notwithstanding, the small size of the these outlet lands and the substantial cooling performance of the trailing edge cooling slots, the thin trailing edges of turbine airfoils nevertheless typically limit the life of those airfoils due to the high operating temperature thereof in the hostile environment of a gas turbine engine.
The pressure ratio of the cooling air to the mainstream gas in the turbine flowpath is typically highest at the trailing edge, so metering the cooling flow to the desired level is often difficult. Film cooling effectiveness must be sufficiently high on the both the slot floor or deck and the lands to maintain acceptable metal temperatures. This is significant challenge on the lands, which must rely on lateral cooling flow migration to reach the top surface.
Accordingly, it is desired to provide a turbine airfoil having improved trailing edge cooling and cooling slots for improving airfoil durability and engine performance. It is also desired to minimize the amount of cooling flow used for trailing edge cooling in order to maximize fuel efficiency of the turbine and the engine. It is also desirable to provide cooling air metering so that blowing ratio is minimized the cooling hole exit to the slot to maintain good slot floor film effectiveness, low cooling flow, and cooling film effectiveness over the land with good flow alignment with land edges along with a smaller land surface area.